The present invention concerns the reduction of solid feedstock comprising metal compounds, such as metal oxides, to form products. As is known from the prior art, such processes may be used, for example, to reduce metal compounds or semi-metal compounds to metals, semi-metals or partially-reduced compounds, or to reduce mixtures of metal compounds to form alloys. In order to avoid repetition, the term metal will be used in this document to encompass all such products, such as metals, semi-metals, alloys, intermetallics and partially reduced products.
In recent years there has been great interest in the direct production of metal by reduction of a solid feedstock, for example, a solid metal oxide feedstock. One such reduction process is the Cambridge FFC electro-decomposition process (as described in WO 99/64638). In the FFC method a solid compound, for example a solid metal oxide, is arranged in contact with a cathode in an electrolytic cell comprising a fused salt. A potential is applied between the cathode and an anode of the cell such that the solid compound is reduced. In the FFC process the potential that reduces the solid compound is lower than a deposition potential for a cation from the fused salt. For example, if the fused salt is calcium chloride then the cathode potential at which the solid compound is reduced is lower than a deposition potential for depositing calcium from the salt.
Other reduction processes for reducing feedstock in the form of cathodically-connected solid metal compounds have been proposed, such as the Polar process described in WO 03/076690 and the process described in WO 03/048399.
While the reduction of solid feedstock to metal in an electrolytic cell comprising a molten salt has been carried out for a number of years on a laboratory scale, it has not proved easy to scale up production to an industrial level.
In a typical electrolytic reduction process the electrolytic cell comprises a cathode, an anode and a feedstock arranged in contact with a molten salt. The salt is heated to a molten state within the cell and during the reduction process the salt becomes contaminated with elements evolved from the feedstock and by reactions with the containment materials and electrodes. When performing an electrolytic reduction using such a cell, the entire cell needs to be heated to a temperature at which the salt is molten, which takes a considerable amount of energy and time. Once the reduction is complete the entire cell including the salt needs to be cooled and energy that has been put into the system to heat the salt is lost.
It is an aim of the invention to provide an improved apparatus and method for the electrolytic reduction of solid feedstock.